The present invention relates to a process for the preparation of a biaxially stretch-oriented film having at least one opaque layer, composed essentially of a propylene polymer, preferably polypropylene, and fillers in a quantity of 10 to 40% by weight relative to the total weight of propylene polymer and fillers. In the process, granules are melted in a screw extruder, are forced through a die and are formed by cooling to give a preformed film which is then stretch-oriented both along the machine direction and transversely perpendicular to the machine direction and is then heat-set.
Films comprising a layer having a filler content of incompatible or hardly compatible organic or inorganic pulverulent materials are disclosed, for example, in German Auslegeschrift No. 2,814,311 and German Offenlegungsschrift No. 3,436,961. The films described in these publications are provided by the addition of the above-mentioned filler with an opaque appearance, show a high surface gloss, are heat-sealable or cold-sealable and, finally, can very readily be imprinted. The opacity of these films is produced during stretching of the coextruded film, the polymer matrix being torn open at the grain boundaries of the filler which is incompatible with the polypropylene. As a result, voids or microcavities are formed across the entire interior of the film, where the refraction of light differs from that on the polymer matrix. Due to the refraction or scattering of light in the region of these cavities, the film has an opaque appearance which is very effective in advertising and therefore desirable.
The films according to the state of the art are produced by known processes. The basic operations of the production process are represented by the process steps of extrusion (melting) of the raw materials for the base layer and covering layers, forming of the melt strands to give a flat film of melt and cooling of this film of melt to give a preformed film. In this case, the density of the preformed film is equal to the density calculated from that of the polymer material and of the filler. The film is then biaxially stretched sequentially, either first in the longitudinal direction and then in the transverse direction, or first in the transverse direction and then in the longitudinal direction, in a known manner (cf. German Auslegeschrift No. 2,814,311). The intervening steps, for example cooling of the film for solidification and reheating of the film to the stretching temperature, and the step following the last stretching, namely heat-setting, are also carried out in a known manner.
In this process hitherto used, the desired quantities by weight of organic or inorganic fillers are added to the polymer during the preparation of the raw material for the layer provided wtih fillers. This is done during granulation of the polypropylene powder, for example, in twin-screw extruders, where other additives, such as antioxidants, antistatic agents and/or stabilizers, are also added to the raw material, each in the optimum quantity.
For the purpose of film production, cylindrical granules having a diameter of 3 to 4 mm and a length of 4 mm, as is also customary for other applications, were employed originally. The processing of these granules having a filler content of more than 10% by weight of the incompatible material, together with the proportion economically required of regranulated material produced from reclaimed material arising during film manufacture, in the single-screw extruders conventional in industry was admittedly quite successful initially, but when the extrusion speed was raised, i.e., the extruder output was increased, fine bubbles appeared in the finished film and made the product useless. It was possible to identify the bubbles as air bubbles.
Surprisingly, these extremely undesirable bubbles were eliminated, for the case of raw material filled with up to 10% by weight, by changing over from cylindrical granules to lenticular granules, the lenticular granules having a diameter of about 5 to 6 mm and a thickness of about 2 to 3 mm. However, it was found that even this processing method is prone to faults since, from time to time at irregular intervals, bubble formation was again and again observed.
The improvement of the processability by the choice of lenticular granules in place of the otherwise conventional cylindrical material can be seen in the increase of the bulk density from 530 kg/m.sup.3 on the average to 570 kg/m.sup.3 at a conventional reclaimed material content of 30 to 40%, and also in an increase in the specific surface area of the packed material. Due to the larger surface area, the air present in the grain passes more rapidly into the grain interstices, when the material is heated in the extruder, from where it can be squeezed off toward the hopper. Of course, owing to the higher bulk density, less air is initially present in the system than in the case of a lower bulk density.
Although it is possible in principle that a removal of air present in the screw space can be carried out--at least partially--by means of so-called venting zones, vacuum hoppers or twin-screw extruders with venting facilities, these equipment modifications first of all make the existing equipment more expensive and, on the other hand, do not guarantee the desired result in every case. In addition, it is conventional and in many cases advantageous under process engineering aspects to carry out the extrusion of polypropylene films in single-screw extruders, preferably in single-screw extruders provided with grooves having a forced conveying action.
Processing of raw material having a content of more than 10% by weight of incompatible or hardly compatible organic or inorganic fillers to give bubble-free films by means of lenticular granules did not, however, succeed in a satisfactory manner, that is to say at throughput rates desirable for the production process. Mainly in the case of films of relatively large thickness (thickness greater than 50 .mu.m), the surface of the films showed a more or less irregular bubble structure which it was not possible to eliminate completely even by optimization of the extrusion conditions (colder or warmer feed zone, preheating, i.e., temperature control with partial venting of the mixture of granules and reclaimed material). Film meeting the specifications and having a filler content of more than 10% by weight of the fillers has not yet been produced by the various processes used according to the state of the art.